The invention relates to the field of undercarriages, and more particularly to the hydraulic circuit for actuating undercarriages.
The undercarriages of aircraft are generally retractable, and for this purpose they include actuators that control the deployment and the raising of undercarriages as well as the opening and the closing of the doors of bays for receiving undercarriages in the raised position.
The prior art is described herein with reference to FIG. 1, which is a diagram of an aircraft undercarriage. The undercarriage 1 comprises a leg 2 hinged to the aircraft about an axis 8 and shown here in the deployed position. A double-acting undercarriage actuator 5 having a main chamber and an annular chamber is coupled to the leg 2 to operate the undercarriage 1 between a raised position and a deployed position. The undercarriage 1 is suitable for entering into a bay that is closed by hinged doors 3a and 3b, each fitted with a double-acting door actuator 4a/4b serving to open and close the doors 3a and 3b. An undercarriage uplock box 6 serves to lock the undercarriage 1 in the raised position, and a door uplock box 7 serves to lock the doors in the closed position. The uplock boxes 6 and 7 carry hooks that are mounted to move between an unhooked position that is stable and a hooked position that is likewise stable. The undercarriage 1 and the doors 3a and 3b include respective rollers that, on being raised or closed, serve to co-operate with the corresponding hooks in order to push the hooks towards the stable hooking positions in which the rollers are held captive by the hooks. The uplock boxes 6 and 7 are fitted with single-acting actuators that, on being actuated, cause the hooks to go from the hooking position to the unhooking position, thereby releasing the corresponding element.
With reference now to FIG. 2 that shows a prior art hydraulic circuit for actuating undercarriages, the door actuators 4a and 4b and the undercarriage actuator 5, together with the actuators of the uplock boxes 6 and 7 are powered by a hydraulic circuit that comprises:                a feed line 10 for feeding hydraulic fluid under pressure;        a hydraulic fluid return line 11.        a first service line 21 of the door selector 14, referred to as the opening line 21, that feeds the main chambers of the door actuators 4a and 4b and allows them to be extended, and thus the doors to be opened;        a second service line 22 of the door selector 14, referred to as the closure line 22, that feeds the annular chambers of the door actuators 4a and 4b and enables them to be retracted, and thus the doors to be closed;        a door selector 14 that enables the door service lines 21 and 22 at the outlet from said selector to be selectively connected to the feed line 10 or to the return line 11;        a first service line 25 of the undercarriage selector 15 referred to as the deployment line 25 that feeds the main chamber of the undercarriage actuator 5 and enables it to be extended, and thus enables the undercarriage to be deployed;        a second service line 26 of the undercarriage selector 15 referred to as the raising line 26 that feeds the annular chamber of the undercarriage 5 and that enables it to be retracted, and thus enables the undercarriage to be raised; and        an undercarriage selector 15 enabling the undercarriage service lines 25 and 26 at the outlet of said selector to be connected selectively to the feed line 10 or to the return line 11.        
The hydraulic circuit also includes a control member 9 that controls the door and undercarriage selectors 14 and 15. The control member 9 receives instructions to raise or to deploy the undercarriages and forwards the appropriate commands to the selectors. The control member 9 controls the deployment and raising sequences and it may also take account of information coming from sensors.
The door selector 14 comprises a movable slide that can take up three positions 14a, 14b, and 14c in which the position 14b is stable while the extreme positions 14a and 14c are reached only if the control member 9 issues a corresponding order:                in position 14a, the selector 14 feeds pressure to the closure line 22 that is connected to the annular chamber of each door actuator 4a, 4b, thereby enabling said actuators to retract and causing the doors 3a and 3b to close, while the other line is connected to the return;        in position 14b, the selector 14 connects both door service lines to the return; and        in position 14c, the selector 14 feeds pressure to the opening line 21 connected to the main chamber of each door actuator 4a and 4b, enabling said actuators to be extended and causing the doors 3a and 3b to be opened, the other line being connected to the return.        
The undercarriage selector 15 is identical to the door selector 14 and comprises a movable slide that can take up three positions 15a, 15b, and 15c, serving respectively to retract, to connect to the return, and to extend the undercarriage actuator 5.
The hydraulic circuit shown in FIG. 2 also has a door unhooking line 23 feeding the actuator of the door uplock box 7. The door unhooking line 23 is connected to the door opening line 21 such that causing the doors 3a and 3b to open also causes power to be fed to the actuator of the door uplock box 7. Thus, the selector 14 can cause the doors 3a and 3b to be opened while simultaneously causing them to be unhooked.
In similar manner, an undercarriage unhooking line 27 feeds the actuator of the undercarriage uplock box 6 and is connected to the undercarriage deployment line 25. Causing the undercarriage 1 to deploy also causes the actuator of the undercarriage uplock box 6 to be powered and thus causes the undercarriage 1 to be unhooked. The selector 15 can cause the undercarriage 1 to be deployed while simultaneously causing it to be unhooked.
The prior art hydraulic circuit is used in application of the raising and deployment sequences as described below.
The undercarriage deployment sequence comprises the steps of:                opening the doors, thereby causing them to be unhooked;        deploying the undercarriage, thereby causing it to be unhooked; and        closing the doors, thereby causing them to be hooked automatically.        
The undercarriage raising sequence thus comprises by symmetry the steps of:                opening the doors, thereby causing them to be unhooked;        raising the undercarriage, thereby causing it to be hooked automatically; and        closing the doors, thereby causing them to be hooked automatically.        
Causing the doors 3a and 3b to be unhooked simultaneously with them being opened nevertheless gives rise to certain difficulties. Opening the doors while they are still hooked leads to a large force being applied to the hook. The hook must therefore be capable of withstanding this force and the actuator of the door uplock box 7 must be capable of going from the hooked position to the unhooked position in spite of the doors pressing against the hooks.
The same difficulties appear when deploying the undercarriage.